This invention relates in general to sewing machines and in particular to a new and useful sewing machine having a device for measuring the pressure exerted by a presser for calibrating the pressure with a reference source.
For some sewing jobs, the normal value of the pressing force of the presser foot cooperating with the sewing machine feed, as adjusted when the sewing machine was assembled, is inappropriate. Thin and pressure sensitive fabrics, for example, require a reduced pressing force, while thick fabrics as well as firm material or fabrics with a smooth surface require a comparatively higher pressing force. For material difficult to work, therefore, it would be advantageous if the pressing force of the presser foot would be adjustable to empirically established values which take into consideration the influences of different parameters such as work thickness, number of plies, type of fabric, and sewing speed.
In view of U.S. Pat. No. 3,055,326 a sewing machine with an adjustable presser system is known in which a setting wheel with control cam means presents several display symbols, e.g. numbers, which appear one by one in a window. While by means of the display symbols the pressing force of the presser foot can be adjusted stepwise, the pressing force, which becomes active at the individual display symbols at the presser foot, depends on the respective spring constants of the spring mechanism and on the friction conditions of the entire presser system. Since thus the pressing forces actually occurring at the same display symbols may differ from machine to machine, set values which take into consideration different parameters determined by tests cannot be transformed with the aid of setting wheels carrying display symbols with the necessary accuracy into corresponding pressing forces of the presser foot even when the set values are indicated in the form of the same symbols as used on the setting wheels.